Conventional small-scale image acquisition systems, such as endoscopes and boroscopes, typically sample an image plane using a bundle of optical fibers that correspond to pixels on a camera detector such as a charge coupled device (CCD). Trying to minimize a system's size using this approach is limited by a number of factors, including the overall diameter of the fiber bundle, the number of pixel detectors on the camera detector, and diffractive properties of light beams. Reducing the diameter of a conventional acquisition device reduces the possible number of pixels, and thus reduces the resolution and/or field of view (FOV) of the device. However, a reduction in diameter and size would enable users to examine areas unreachable by currently designed endoscopes, reduce collateral damage to tissue, and enable integration of imaging with other functional devices such as therapy devices.
Similarly, many small-scale image display systems, such as head mounted displays (HMDs), beam light from an optical fiber onto deflectable mirrors or rotating polygonal mirrors to produce an image on an image plane. This approach also has many size limitations. For instance, light beams of less than 3 millimeters (mm) are impractical for displays using mirrors, because mirror scanners and grating deflectors must be significantly larger than the light beam diameter to avoid clipping the beam or adding diffraction. Reducing the diameter of a conventional display device reduces the possible number of pixels, and thus reduces the resolution and/or field of view (FOV) of the device. However, a reduction in diameter and size would enable construction of more comfortable HMDs, and enable integration of display with other functional devices.
An older type of scanning image display system includes an electromechanical modulator. The modulator comprises a full width array of closely spaced fiber-like reflectors which deflect when a voltage potential is applied. The voltage potential is selectively applied to the reflector in accordance with an image signal. This technique requires a very complicated circuit to control the overall deflection of the reflectors and the overall size is quite large.
As one practical application, minimally invasive medical procedures (MIMPs) has increased the demand for small diameter systems that result in less tissue damage and trauma, faster recovery times, and lower risks to the patient. Typically, instruments used by practitioners of MIMPs include several different discrete systems for optical imaging, monitoring, maneuvering, sizing, diagnosis, biopsy, therapy, surgery, and non-visual monitoring/sensing. It would be preferable to combine the functions provided by these instruments in a single compact device to reduce the number of surgical ports that are currently required for a plurality of single-function tools. By employing an integrated multi-functional tool so that only one small port is used, the risks associated with repeatedly removing and inserting surgical tools can be dramatically reduced. Since most MIMPs require the practitioner to constantly monitor the procedure visually, optical imaging is considered a requirement for any fully integrated system for MIMPs. Thus, an appropriate multifunction instrument will most likely include an optical imaging system, and the imaging system should be compact so that it can be integrated with one or more diagnostic, and/or therapeutic tools.
The current tools used for MIMPs cannot readily be integrated with an optical imaging system without increasing the size of the resultant instrument to an excessive degree. All currently available commercial optical imaging systems that include a maneuverable flexible shaft must maintain a certain size (diameter) in order to preserve image quality. As indicated above, currently available flexible scopes cannot be made smaller than this limit unless image field-of-view (FOV) or resolution is sacrificed. Also, currently available imaging systems typically use an external light source to generate light, and use an optical waveguide to direct the light to an ROI within a patient's body. Although imaging and some diagnostic capability can be integrated into existing scopes, such as standard tissue imaging in combination with fluorescence for early detection of cancers, the optical systems of current flexible scopes are not sufficiently small to provide integrated diagnoses and therapies at the required degrees of performance, size, and price that will be demanded in the future by medical practitioners.
Presently available flexible scope designs use either a bundle of optical fibers (optical waveguides) and/or one or more cameras having an array of detectors to capture an image. Thus, the diameter of these flexible scopes employed for remote imaging cannot be reduced to smaller than the image size. Even if one ignores additional optical fibers used for illumination of an ROI, the scope diameter is therefore limited by the individual pixel size of a camera or by the diameter of optical fibers used to acquire the image. Currently, the smallest pixel element is determined by the size of the end of an optical fiber, which has a minimum core diameter of about 4 μm. To propagate light through an optical fiber, a surrounding cladding layer is required, increasing the minimum pixel size to more than 5 μm in diameter. If a standard video graphics adapter (sVGA) image is desired (e.g., with a resolution of 640×480 pixels), then a minimum diameter required for just the imaging optical fiber is more than 3 mm. Therefore, to achieve scopes with less than 3 mm overall diameter using current technologies, resolution and/or FOV must be sacrificed by having fewer pixel elements. All commercially available scopes suffer from this fundamental tradeoff between high image quality and small size.
Currently available scopes also suffer from poor control mechanisms. Some optical systems use an optical fiber and camera at a tip of a flexible scope to illuminate a ROI and acquire an image. The fiber and camera are manually controlled by the practitioner positioning the tip of the flexible scope. Other optical systems use a resonant fiber that is actuated into resonance with one or more nodes to produce a desired illumination spot. Although these systems actuate the fiber, such systems can not precisely control the position of the fiber tip without adding material to the fiber scan system and increasing the diameter and/or rigid-tip length. Other optical systems deflect or move mirrors to position the light beam rather than move the waveguide. However, as discussed above, mirrors must be larger than the light beam diameter to avoid clipping the beam or adding diffraction. Thus, the mirrors must be larger than the waveguide, thereby increasing the overall size of the instrument.
Some microscopes actuate a cantilever waveguide for near-field imaging. However, near-field systems have a very limited FOV (e.g., typically less than 500 nanometers), and a light-emitting tip must be positioned within nanometers of the target. Near-field systems are based on emitting light through a microscopic aperture with dimensions smaller than the wavelength of visible light. The emitted light reflects off the closely positioned target and is detected before the light has time to diffract and dissipate. A near-field system may be useful for imaging individual cells or molecules, but is not suitable for most medical procedures and other dynamic applications which require a FOV of at least a micron and can not be dependant on precisely positioning a tip within nanometers of the target. Using larger wavelengths to provide a suitable FOV with a near-field system would still require a substantially larger imaging system, which could not be integrated into a multi-function instrument. As an alternative, some microscopes actuate a cantilever waveguide for confocal microscope imaging. However, simple confocal systems are limited to single wavelength operation, which does not enable color imaging or display.
Thus, it would be desirable to reduce the imaging system for the purpose of reducing the overall size of an instrument used for MIMPs and other applications. To currently perform diagnostic or therapeutic MIMPs, one or more separate instruments are used within the FOV of a standard endoscopic imager, and any additional separate instrument must often be held and maneuvered by a second medical practitioner. Typically, the second instrument provides a high intensity point source of light for optical therapies, a hot-tipped probe for thermal therapies, or a trocar used for mechanical cutting. The second instrument is moved to the surface of the tissue and usually moved within or across the surface of the tissue, covering the area of interest as the tool is scanned and manipulated by hand. These secondary instruments are often inserted into the patient's body through a separate port, and thus, while being used, are viewed from a different point of view in the visual image. Furthermore, the therapeutic instrument often blocks the practitioner's direct view of the ROI with the imaging tool, making highly accurate therapies quite difficult for the medical practitioner to achieve. Significant amounts of training and practice are required to overcome these difficulties, as well as the capability to work with a reduced sense of touch that is conveyed through the shaft of an instrument having friction and a non-intuitive pivot at the point of entry. Thus, to work effectively with current imaging and therapeutic technologies, the practitioner of MIMPs must be highly trained and skilled.
Clearly, there is a need for an imaging system that is small enough to be integrated with diagnostic and/or therapeutic functions to create an instrument that is sufficiently intuitive to use as to require little training or skill. Similarly, a small, integrated display system would greatly improve mobility for a head mounted display and enable very localized display of images. Ideally, an image acquisition or display system should integrate a light source, an actuation system, a position sensing system, light detectors, and a local control system, yet be smaller than currently available systems. Despite its small size, the integrated system should still be capable of providing a sufficient FOV, a good image size, and high resolution. The integrated system should also enable a practitioner to ensure that therapy can be administered to the ROI imaged within a patient's body. Currently, no integrated system is small enough to provide these capabilities and cannot be easily modified to provide such capabilities.